The present invention relates to a semiconductor device; more particularly, to a method for fabricating a CMOS image sensor having a plurality of unit pixels which are capable of reducing a dark current.
As is well known, an image sensor is an apparatus for sensing a light beam reflected from an object to generate an image data. Especially, an image sensor fabricated by using a complementary metal oxide semiconductor (CMOS) technology is called a CMOS image sensor.
Generally, the CMOS image sensor includes a plurality of unit pixels. Each of the unit pixels also includes a light sensing element and a plurality of transistors. The light sensing element such as a photodiode senses incident light beam to generate photoelectric charges corresponding to an amount of the incident light beam. The transistors perform switching operations to control a transfer of the photoelectric charges.
FIG. 1 is a cross-sectional view showing sequential steps of fabricating a conventional unit pixel contained in a CMOS image sensor.
Referring to FIG. 1, a P-type well 12 and a field oxide layer 13 are formed on a semiconductor substrate 11, and a PN junction region 17A and 17B is formed in the semiconductor substrate 11 to thereby provide a photodiode 17. Then, a floating junction region 18A, to which photoelectric charges generated in the photodiode 17 is transferred, is formed within the semiconductor substrate 11.
Then, a transfer transistor TX for transmitting the photodiode to the floating junction region 18A and a reset transistor RX for resetting the floating junction region 18A are formed on the semiconductor substrate 11. A drive transistor DX for amplifying a voltage level corresponding to the transferred photoelectric charges and a select transistor SX for outputting amplified voltage level as the image data are formed on the P-type well 12. At this time, the reset transistor RX and the drive transistor SX are commonly coupled to a common junction region 18B, and an impurity junction region 19 of a lightly loped drain (DLL) structure is formed between the drive transistor DX and the select transistor SX. Also, spacers 20 are formed on sidewalls of each transistors TX, RX, DX and SX.
Then, pre-metal dielectric (PMD) layers 21 and 22 are formed on the transistors TX, RX, DX and SX, and interlayer insulating layers 23, 24 and 25 are formed on the PMD layer 22.
Then, the PMD layers 21 and 22 and the interlayer insulating layers 23, 24 and 25 are selectively etched and first metal lines M1, M2 and M3 and a second metal line M4 are formed thereon, respectively. The first metal lines M1, M2 and m3 and the second metal line M4 used to connect the transistors TX, RX, DX and SX with an external elements are formed with staked layers of Ti/Al/TiN.
Then, a passivation layer formed with an oxide layer 29 and a nitride layer 30 is formed on the second metal line M4.
Then, a color filter array (CFA) operation is carried out to thereby form a color filter 31, and a dyed photoresist 32 is formed on an entire structure. Then, a microlens 33 is formed above a portion where the color filter 31 is formed.
At this time, a number of surface energy states exist in a forbidden band due to a dangling bond of a lattice structure. The surface energy states result in a recombination of carriers. As a result, a leakage current is increased and a breakdown voltage of the image sensor may be influenced. That is, an undesired dark current is flowed so that a reliability of the image sensor is degraded.
It is, therefore, an object of the present invention to provide a method for fabricating a CMOS image sensor having a unit pixel which is capable of reducing a dark current.
In accordance with an aspect of the present invention, there is provided a method for fabricating a CMOS image sensor, wherein the CMOS image sensor includes a plurality of unit pixels, the method comprising the steps of: a) providing a semiconductor structure, wherein the semiconductor structure includes a photodiode and peripheral elements formed on a semiconductor substrate; b) forming an insulating layer on the semiconductor structure; c) forming a hydrogen containing dielectric layer on the insulting layer; d) diffusing hydrogen ions contained in the hydrogen containing dielectric layer into a surface of the photodiode, thereby removing a dangling bond; and e) removing the hydrogen containing dielectric layer.